Retention of geometry

The palladium/copper-cocatalyzed coupling of the readily available trisubsti-tuted alkenyl(phenyl)iodonium triflates 77 with alkynyl- and alkenylstannanes proceeds under exceedingly mild conditions with retention of geometry of the alkenyl ligand of the iodonium salt (Scheme 35) [60]. 

These reagents are usually prepared by treatment of a phosphine with an alkyl halide and successive addition of base, like BuLi or NaHMDS. Reaction with the ketone takes place by attack of the carbanionoid carbon of the ylide form on the electrophilic carbon of the carbonyl group via a four-membered heterocyclic transition state, the oxaphosphetane 35. The driving force for this transformation is provided by formation of the very strong phosphorus-oxygen bond. Subsequent collapse of the oxaphosphetane furnishes the desired exomethylene group under retention of geometry. 

Vinyl silanes react with electrophiles in a highly regioselective process in which the silicon is replaced by the electrophile at the ipso carbon atom. The stereochemistry of the vinyl silane is important because this exchange usually occurs with retention of geometry as well. Consider the reaction of the two vinyl silanes derived from phenyl acetylene with the simple electrophile D+. Deuterons are chemically very similar to protons but are, of course, distinguishable by NMR. 

CoH(N2)(PPh3)3] has been extensively studied 409 it has the distorted trigonal-bipyramidal structure (93 Table 32). The end-on coordinated N2 is reversibly displaced by H2, C2H4 and NH3, and is irreversibly displaced by CO and PF3 with retention of geometry (94), It also reacts with formaldehyde to give the trans complex (95) (structure, Table 32). 

Photolysis (436 and 254 nm) leads to both halide and py labilization from franx-[Rhpy4X2]+ (X = Cl, Br).796 The efficiencies of Cl" and py release are comparable (upon d-d excitation at 436 nm), but are uncharacteristically low for photoinduced Rh—Cl bond cleavage. Bromide release dominates over py release upon 254 nm excitation of franx-[Rhpy4Br2]+. The photoreactions are presumed to proceed with retention of geometry.796... 

In boriozirconocene aUcenes, the chemistry of Csp -Zr and Csp -B bonds differ considerably and this feature allows a sequential route to substituted aUcenes. Furthermore, cleavage of the two types of bonds generally occurs with retention of geometry. On the other hand, heating of aUcynezirconocene complexes with 9-BBN, to 80-90 °C, affords a bimetallic complex (95) showing a planar-tetracoordinate carbon and an wo-BBN structure. 

The question that must be asked is why there are not many more known reactions in which retention of geometry is not featured. There are probably a number of reasons. One explanation offered is that pseudorotation at d species is slow compared with ligand replacement... 

A more versatile palladium-catalyzed formylation of organic halides takes place using tributyltin hydride and carbon monoxide (equation 7). The reaction works for a variety of substrates — aryl, benzyl and vinyl iodides, vinyl triflates and allyl halides. Reaction conditions are mild (1-3 bar CO, 50 °C), and a variety of functional groups can be tolerated. With unsymmetrical allyl halides formylation is regio-selective, taking place at the less-substituted allylic position with retention of geometry at the allylic double bond. 

Stereochemical control of the substitution reaction in cis- and frans-[CoCl(en)2(OH)]+ via different transition state geometries. The square pyramidal intermediate offers only one choice to the entering group and 100% retention of geometry, the trigonal bipyramidal intermediate offers three entry points around the triangular central plane, two leading to cis entry and one to trans entry, for a 2 1 ratio. 

The dipolar addition of (diazoalkyl)phosphonic esters to (isopropenyl)phos-phonates gives the di(phosphonates) (188) in their trans form. When these are heated, a mixture of cis and trans cyclopropanes (189) is formed, evidently via intermediates having a substantial life-time. By contrast, the photolysis of the esters (188) yields (189) with complete retention of geometry, presumably by way of intermediates having very short life-times, e.g., biradicals. ... 

Retention of geometry, perfect chirality transfer, and high reactivity have been observed by the reaction of the chelated Zn enolate of amino acid ester 53 even when PPhs was used [23]. In addition, the non-stabilized enolate 53 was found to be very reactive. Reaction of the allylic carbonates 54 and 56 with the enolate 53 gave 55 and 57 with perfect chirality transfer and high diastereoselectivity. The carbonates and the enolates are highly reactive and the reaction starts even at —78 °C. Lower selectivity was observed by the reaction of the corresponding allylic acetate. 

A route to allyl vinyl thioethers with defined E- or Z-geometry at the vinyl double bond has been reported (Scheme 46) the key step is reaction of a vinyl alanate (89) or (90), produced stereoseleCtively, with an allyl thiosulphonate and proceeds with retention of geometry at the alane carbon. The products are potential substrates for thio-Claisen rearrangements. 

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